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General Category >> Rubber Side Down! >> Will someone grade my "carb theory" exam?
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Message started by Sandy Koocanusa on 07/05/08 at 20:34:34

Title: Will someone grade my "carb theory" exam?
Post by Sandy Koocanusa on 07/05/08 at 20:34:34

I'm struggling with getting my jetting correct. I have read for hours about this, and I'm not going to ask the usual questions, I promise. I just need someone to tell me if I'm making up some of what I think I know, or if it is actually correct.

A brief history: When my bike was stock, it ran just fine. I read all of the articles and got rid of any backfiring issues by simply adjusting the idle air screw. Then I got creative (rarely a good thing with me). I took out most of the stuffing from my stock muffler. It sounded good, but it ran lousy. Way to lean and no more adjustment available. So I increased the size of the pilot jet from 47.5 to 50. With that change, the bike ran waaaay to rich. I turned the screw all the way in and backed it out about 1/8 turn, but it was no good. Loud booming backfires on decel, gold pipe, and about a 20% reduction in mileage.

So, I got to snooping some more and decided that maybe Oldfeller and Yoda were on to something. Last night I ripped up an air filter and put together a Nufoam and canola oil replacement. I left the door off the air box, too. It's too soon to tell for sure, but at least the backfiring seems to have stopped. I'll check the mileage in a couple of days.

Here's where I'm unsure... I THINK that when I twist the throttle, more air is allowed into the carburetor. I THINK that this increase in air draws more fuel through the jet (which allows, but does not cause, the fuel to pass). So I THINK that with proportionally larger jetting and air intake, I should be able to achieve the same power and mileage with less throttle as I had with the small jet and congested intake. I think that increasing all these things should allow more power if I choose it, but that I should still be able to drive economically by twisting the throttle less.

If 1/2 throttle allows 1 drop of gas and 14.7 drops of air into the engine with a small jet, then 3/8 throttle should allow the same 1 drop and 14.7 drops in with larger jet and air intake, right? Same amount of fuel/air, same power, same mileage?

Title: Re: Will someone grade my "carb theory" exam?
Post by Steve M on 07/06/08 at 04:10:10

Hi Sandy,
the idle screw is only realy effective up to about 1/4 throttle.
the needle on throttle slide and it's position, (has 3 clip positions that can raise or lower it). takes over for mid range and the main jet for larger throttle openings.
That is a slightly simplistic view really as in truth they "overlap".

I think best thing to do is adjust the idle screw for a healthy tickover, and don't fit a larger idle jet unless you have to wind screw out more than 3 turns from fully in.
Leave needle jet on throttle slide well alone if bike was running fin before, and fit a larger main jet if bike runs lean.
Mains jets are cheap, available and easy to change.
Obviously what size you end up finding works best depends on how much the bikes "breathing" has changed.
I live in Uk and know we have different words for some things but hope you get my drift!

cheers

Steve

Title: Re: Will someone grade my "carb theory" exam?
Post by Sandy Koocanusa on 07/07/08 at 20:58:02

I know that all the charts and graphs agree with what you say, Steve. The trouble is, my bike wants to argue. I'm running a 155 main jet. The bike came that way stock. It ran toward lean, even three turns out and before I fooled with the exhaust. The new pilot jet immediately changed the way it was running. Now it has all the symptoms of over-rich, despite having the screw almost bottomed out.

The temperature here jumped from about 50 to about 90 right around the time I changed the jet, which I know adds to the problem.

I need to mark my throttle grip and really pay attention to where it is at highway speed, but it feels like I'm barely turning the grip when I'm cruising. There is A LOT of twist left at 65 mph. I've never run the thing out to see what that would translate to in speed. I know it'll zip right up to 85.

I'm still not clear on what all it implies, but Savage Greg recently diagnosed my bike as having California Modelitis. I know that means it's sucking fumes from somewhere and putting them back into the mix. I wonder if that affects carburetion? I don't believe I've seen it mentioned anywhere. Hmmm...